The present invention relates to a thermal expansion absorber for accommodating an increase in volume of a heat conductive fluid in sealed electric motors and in particular relates to a pressure-variable sealed volume such as a rubber tubing, sealed at both ends, filled with a compressible heat-conductive fluid such as helium, and disposed in the sealed chamber of a motor stator case to accommodate expansion of a fluid such as oil which thermally conducts the heat internal of the motor to the external casing to eliminating hot spots within the motor.
It is well-known in the prior art that the stator of certain motors, such as those which drive certain pumps, has a chamber therein which is filled with oil for thermal conductivity. During operation of the motor, as the temperature of the motor rises, the temperature of the oil increases thereby conducting heat from the internal windings of the motor to the external casing and also flowing across any hot spots to prevent excessive localized temperatures. Of course, the volume of the oil increases as its temperature increases. Some means must be provided to absorb this expansion since the oil is essentially non-compressible. The prior art utilizes several different means of accommodating the expansion. One apparatus which is used is a relief valve mounted on the casing and when the oil expands beyond the capability of the stator chamber to hold it, it simply escapes through the relief valve. However, oil escaping from the relief valve is a nuisance since it runs down the outer casing of the motor and collects dirt, dust and otherwise provides problems for maintenance personnel.
Another apparatus requires an auxiliary chamber which is coupled to the stator chamber such that if the oil expands beyond its capacity in the stator chamber, it can overflow into the auxiliary chamber. This of course is also undesirable since the added chamber is not only expensive but the added components require additional attention and maintenance.
Another means presently used is to provide a pocket of air in the sealed stator chamber into which the oil can expand. However, the location of such an air pocket can vary during operation, and then can be situated surrounding a hot spot in the windings of the motor. Since no fluid, other than air, is around the windings in those areas to carry away the heat such uncooled hot spot can cause failure of the motor.
The present invention overcomes the disadvantages of the prior art by providing a compressible sealed container, such as a length of resilient rubber tubing sealed at both ends, filled with a compressible thermally conductive fluid having a thermal conductivity approximately equal to that of the oil in the stator chamber. Such fluids include gases such as helium or hydrogen. At least one such container is placed in the stator chamber before final assembly of the stator casing. As the oil is warmed and expands in the stator casing, the fluid in the container or tube is compressed thereby absorbing the expansion, somewhat increasing the pressure throughout the case. By use of the compressible fluid in a container such as a sealed resilient rubber tubing, the location of the fluid in direct contact with the windings is distributed and controlled, and therefore maintains the heat conductive liquid in contact with the windings to eliminate the hot spots. Since the container is sealed, the compressible fluid is prevented from leaking onto the case even during a long period of time.
Thus it is an object of the present invention to provide a means for thermal expansion of the fluid in the stator chamber of the motor without the use of relief valves, auxiliary air chambers, or the creation of hot spot winding failures.
It is a further object of the present invention to provide a compressible sealed container filled with a compressible heat conductive fluid for thermal conductivity and placing at least one of these container in the stator chamber before final assembly of the stator so that expanding oil within the stator chamber compresses the fluid in the container thereby absorbing the expansion.
It is also an object of the present invention to form said sealed container of a length of rubber tubing sealed at both ends.
It is still another object of the present invention to fill the resilient container with helium as the compressible thermally conductive fluid. Helium is approximately equal in thermal conductivity to the oil in the stator chamber.
It is also another object of the present invention to provide means for mounting a plurality of concentric resilient tubes in each end of the stator chamber, the concentric tubes encircling and abutting each other.